1. Field of the Invention
The invention concerns a device and method for analyzing particles suspended in a solution. This invention relates, in particular, to the discrimination of noise signals in an electronic device for the analysis of particles by means of which particulate systems can be studied according to the Coulter detection principle. The invention especially applies to blood analysis.
2. Description
The Coulter detection principle is described in U.S. Pat. No. 2,656,508. According to this principle, a liquid electrolyte containing particles in suspension is passed from a first container to a second container through a microscopic passage or orifice arranged in an insulating wall, the orifice being placed in an electrical field formed by the application of electrical energy to electrodes immersed in the electrolyte and placed on either side of the orifice, such that when a microscopic particle in suspension in the electrolyte passes through the orifice a momentary variation in the electrical impedance of the electrolyte measured between the electrodes is produced. This variation in impedance derives part of the excitation energy in the associated electrical measuring circuits and produces an electrical pulse in these circuits. On condition that the path of the particle coincides approximately with the longitudinal axis of the orifice, the amplitude of this pulse is a fairly accurate indication of the volume of the particle. Counting the pulses produced permits a counting of the particles that pass through the orifice up to frequencies of several thousand per second.
In principle, in a particle counter functioning according to the Coulter principle, each pulse is counted, its amplitude is recorded, and the whole of the information obtained in this way is presented in the form of a histogram, which shows the distribution of particles according to their volume. In a device for the analysis of blood particles a certain number of parameters that are important for medical diagnostics (e.g. the hematocrit value) are calculated from the information contained in such a histogram. It is therefore important that this information should be as accurate as possible. If all of the pulses generated by the Coulter principle are considered, the histogram contains partly erroneous information, because among the pulses generated there are on the one hand electrical noise signals and on the other hand pulses corresponding to the passage of particles the trajectory of which is clearly away from the longitudinal axis of the orifice. The amplitude of the latter type of pulse is not representative of the volume of the corresponding particles.
A relatively long orifice could be used in order to reduce the number of the latter type of pulses. However, this has two major disadvantages: on the one hand such an orifice becomes blocked up more frequently and on the other hand counting errors occur much more frequently due to the fact that particles which appear simultaneously in the orifice are incorrectly counted as a single particle. In order to avoid these disadvantages it is therefore advantageous to use an orifice of relatively short length, i.e. a length equal to or less than its diameter.
With regard to the noise signals which in practical applications accompany the pulses generated by the Coulter principle it is interesting to note that it is rather difficult to detect them and to separate them from the useful pulses, as among the useful pulses there are pulses of amplitude comparable with certain noise signals. In addition, noise often affects the edges of particle pulses and thus impedes their discrimination by means of amplitude thresholds. In order to solve this problem it has already been proposed to eliminate all pulses of which the rise time is less than a reference value (French Patent published under No. 2,146,819). Nevertheless, the continuously growing need for greater accuracy and a diversified commercial use of particle analyser devices has led to the conclusion that the discrimination of noise signals must be improved.